Scandinavian ice sheet cracks ancient climate change puzzle

A Museum researcher's 'midge thermometer' innovation has helped explain why the Earth was plunged into a mini ice age almost 13,000 years ago.

Scientists have shown that a melting ice sheet in Scandinavia contributed to the Younger Dryas - a roughly thousand-year-long cold snap that took place between 12,880 and 11,650 years ago.

The abrupt cooling was previously thought to have been caused mainly by the thawing of a huge ice sheet that covered much of modern-day North America.

The team behind the study - from Stockholm University, Plymouth University and the Natural History Museum - analysed fossilised midges and moisture-sensitive molecules found in sediment at the site of an ancient lake in southern Sweden.

Because different midge species thrive at specific temperatures, their remains can be used as an accurate indicator of changing climatic conditions millennia ago.

Museum researcher Steve Brooks, who developed the midge thermometer used in this study, says: 'The assemblage of midge species when compared with modern records enables us to track how, after an initial warming of up to 4°C at the end of the last Ice Age, summer temperatures plummeted by 5°C over the next 400 years.'

This more nuanced model of the Younger Dryas could help scientists better understand what is happening to the Earth's climate today.

'It is important to understand the causes of the Younger Dryas because this tells us how different forces interact to change the climate,' Brooks says.

'This knowledge is essential if we are to accurately predict the timing and magnitude of future climate change.'

The remains of midge heads stored in lake sediment. The team used similar samples to reconstruct temperature records for the period leading up to the Younger Dryas.

A great global cooling

The Younger Dryas mainly affected the northern hemisphere and is the most recent example of a large-scale climate shift.

It interrupted the gradual warming that had been happening since the end of the last Ice Age (roughly 14,000 years ago), briefly ushering in a period of colder, drier weather.

Experts think the cooling saw forests in the northern hemisphere replaced by treeless areas of permanently frozen ground, and possibly the decline of human populations in North America.

The reduction in rainfall in the Middle East during the Younger Dryas is thought to have helped stimulate the beginnings of agriculture, forcing tribes in the Levant to systematically cultivate crops for the first time in human history.

The climate conundrum

While the Younger Dryas' effects are relatively well known, the exact causes have proved harder to pin down.

Previous theories have incorporated a possible comet impact around 13,000 years ago, as well as the eruption of a marine volcano in modern-day Germany.

But the conventional explanation argues that the gradual warming preceding the Younger Dryas led to the melting of a huge ice sheet covering much of North America (the Laurentian ice sheet).

This melting in turn saw many thousands of cubic kilometres of freshwater flood into the North Atlantic Ocean (for comparison, Loch Ness in Scotland contains just over seven cubic kilometres).

The deluge of cold freshwater would have shut down the process that carried warm water towards Europe by the Gulf Stream, causing an abrupt cooling of the North Atlantic and other parts of the northern hemisphere.

The theory has attracted widespread support, but the evidence linking the North American ice sheet melting and subsequent climatic changes is patchy at best.

In particular, conventional climate model predictions based on the North American ice melting failed to match up with Brooks' midge thermometer temperature reconstruction.

'The temperature reconstruction and highly resolved dates of the new record indicated the models' failure to predict the timing of the temperature trends accurately, showing that additional climate forces were required to explain this,' he says.

A Scandinavian shift

Using data gathered at the site of an ancient lake in southern Sweden known as Hässeldala Port, the team devised a new model, which incorporated the melting of an ice sheet in Scandinavia (the Fennoscandian ice sheet).

The new model shows that the North Atlantic Ocean and climate would have been highly sensitive to floods of meltwater coming from the Scandinavian ice sheet.

Predictions based on the new model were a much better match for Brooks' temperature reconstructions, as well as data gathered in separate studies of ice core samples from Greenland.

Francesco Muschitiello of Stockholm University, lead author of the study, says that the introduction of the Scandinavian ice sheet into the model proved decisive in reconciling the conventional theory with recorded data.

'The melting of the Scandinavian ice sheet is the missing link to understanding current inconsistencies between climate models and reconstructions, and our understanding of the response of the North Atlantic system to climate change.'

The new model could even help researchers understand how the current melting of the Greenland ice sheet is likely to affect the climate in the UK and northern Europe, since the freshwater is once again entering the North Atlantic.

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